Due to the low material cost and ease of processing, polymeric materials are widely used in opto-electronic components. An ongoing objective is to replace inorganic glasses that are known to be “fragile”, “heavy” and “hard for machining”. Polymeric materials, however, have optical characteristics that are process dependent, especially birefringence. All optical grade polymers are transparent and amorphous. When amorphous polymers are processed into a desirable shape, they are not optically isotropic, unlike the inorganic glasses. That is, the three indices of refraction, nx, ny and nz, are not equal. This is due to polymer-chain orientation that is unique to polymers. Thus, given a process condition, the observed optical anisotropy depends on the degree of polymer chain alignment. Polymer molecules have intrinsic birefringence Δnint that is determined by factors, such as the polarizabilities of functional groups and their bond angles with respect to the polymer chain. The polymer products have extrinsic birefringence (in-plane or out-of-plane) that is different from the intrinsic birefringence and that is strongly process dependent. Depending on the application, the birefringence has to be controlled to meet the application requirement. In many cases, it is desirable to have substantially low birefringence or phase retardation in both the in-plane and out-of-plane directions.
In the optical disk application such as Compact Disk (CD) and Digital Video Disk (DVD), the substrate materials must satisfy conditions such as, 1) high transmission, 2) low humidity permeation, 3) dimensional stability and 4) low birefringence. Typically, the reading of optical disks involves the detection of slight changes n the polarization state or a change in the intensity of the reflected light from a disk surface. Thus, the birefringence in the disk substrate will have detrimental effects on the readout, such as read-error or noise. Optical disk substrates are manufactured by injection molding of polymers. Polycarbonate (PC) has been widely used for substrates for CD and DVD. It has high transmission, high dimensional stability against heat and humidity, and high mechanical strength. PC, however, has relatively high intrinsic birefringence Δnint. The process of injection molding generates alignment of polymer chains. Thus, a polymer with high intrinsic birefringence, such as PC, is prone to generate unacceptable levels of in-plane retardation Rin and out-of-plane retardation Rth. In order to prevent this problem, one typically adjusts the molding conditions, such as temperature and flow-rate. This optimization of process conditions has been successfully applied to significantly reduce the Rin through the reduction of Δnin. In some cases, the in-plane birefringence Δnin for normally incident light can be made as low as 1˜3×10−5. On the other hand, the out-of-plane birefringence Δnth is typically negative and with the optimized molding process the value is −6˜−5×10−4. Even though the value of Δnth is small, the corresponding phase retardation for obliquely incident light is not negligible due to the substantial thickness of substrate, ˜1 mm. Thus, the light incident on the substrate at an oblique angle φ (measured from the substrate normal direction) will suffer a phase retardation that scales as φ2 for small φ. In some cases, the total phase retardation, taking into account reflection, at φ=30° can reach as much as −150 nm.
In typical Liquid Crystal Displays (LCDs), a liquid crystal cell is situated between a pair of polarizers. Incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the second polarizer. Typical polarizers used widely for liquid crystal displays (LCDs) have a structure such that absorptive polarizing layer (e.g., iodine dye absorbed Polyvinyl Alcohol (PVA) layer) is sandwiched between the triacetylcellulose (TAC) substrate. TAC is widely used for polarizer manufacturing partly because of its low Δnint. For a typical un-stretched TAC, the Δnin is in the order of 5×10−5. Thus TAC with 100 μm thickness has Rin˜5 nm. This amount of phase retardation is not significant and light linearly polarized by the PVA layer essentially remains linearly polarized going through the TAC layer., However, this is true only when light is normally incident to the plane of the polarizer. Most of the TAC substrates are known to have negative Δnth of the order ˜−5×10−4. That would give Rth˜−50 nm. This out-of-plane phase retardation Rth is responsible for the change in the state of polarization for obliquely incident light. It is favorable to have finite negative Δnth in TAC substrates for some modes of LCDs. This is because of the fact that the negative Rth can compensate positive Rth of the liquid crystal molecules that are aligned perpendicular to the liquid crystal cell plane. However, negative Δnth of TAC has a detrimental effect in the LCD mode where the liquid crystal remains essentially parallel to the plane of the cell. This is the case for In-Plane-Switching LCDs, in which liquid crystal molecules rotate while remaining substantially parallel to the plane of the cell.
In a typical backlight LCD, the backlighting assembly contains several optical films that improve the light distribution and polarization before reaching the liquid crystal cell. This backlighting assembly 201 is illustrated in FIG. 2. Light exiting the backlight, 203, first encounters optical films that improve light distribution in the display, such as, diffusing films, 205 and brightness enhancement films, 207. Light is then incident on a reflective polarizer 209 that contains a substrate, 211, and a polarizing layer, 213, which transmits one polarization state and reflects the other polarization state. The next component in the optical path is the absorptive polarizer 215, which contains a bottom substrate, 217, an absorbing polarizing layer, 219, and a top substrate, 221. The transmission axis of the absorptive polarizer and that of the reflective polarizer are parallel. Ideally, the polarization state that is transmitted by the reflective polarizer 209 is the same polarization state transmitted by the absorbing polarizer 215. The optical stack between the backlight 203 and the reflective polarizer 209 recycles the polarization state that is reflected. The polarized light incident on the absorption polarizer 215 must be substantially linearly polarized so that light is effectively transmitted and not absorbed. As stated earlier, typical absorption polarizers contain TAC as a substrate 217, 221 on either side of the absorbing polarizing layer 219. Negative out-of-plane birefringence of TAC used as the bottom substrate 217 converts the linearly polarized light, incident on the absorption polarizer 215, to elliptically polarized light. The polarizing layer 219 will then absorb a portion of the elliptically polarized light. Thus, decreasing the light through put of the display. To have the most light through put, the bottom substrate 217 between the reflective polarizer 209 and the absorptive polarizing layer 219 must have small Δnth and Rth.
As mentioned before, careful adjustment of the process can significantly reduce the Δnin, thus the Rin of the polymeric substrate. It is conceivable that additional optimization of the processing condition would further decrease the residual negative Δnth. However, it increases the manufacturing cost. Alternative method is to form a multilayer. That is to dispose an overlayer with positive Rth on the polymeric substrate having negative Rth. This process provides an optical multilayer that has low Rth (−30 nm<Rth<30 nm) for wavelength λ in the range 400 nm<λ<700 nm.
Several methods of generating a layer with non-zero Δnth thus Rth have been known.
As is well known to those who are skilled in the art, liquid crystals that is uniformly aligned perpendicular to the substrate generate positive Δnth if Δnint of liquid crystal is positive. Polymerizable liquid crystal, such as the one disclosed in U.S. Pat. No. 6,261,649 gives perpendicular alignment. However, liquid crystal compounds generally have a high cost and creating a uniform alignment of liquid crystals in large manufacturing scale is complicated and not trivial. In some cases, it requires photo-polymerization process in order to freeze the perpendicular alignment, adding extra process and cost.
Li et al. (Polymer, Vol. 37, Page 5321–5325, 1996) describe the process of generating the non-zero Rth by spin-coating polyamides on a transparent substrate. The random orientation of polyimide polymer chain is generated. The disclosed process is simple coating of polymers. However, the resulting Δnth and Rth are negative. Therefore, the method only enhances the negativity of the Δnth of the polymer substrates described above.
With process optimizations, it is difficult to obtain a polymer substrate with sufficiently small Rth. Also, the prior art fails to provide a simple method to generate a polymer layer with positive Δnth, thus making the manufacturing process for the polymeric multilayer with low Rth difficult. Therefore, it is a problem to be solved to provide a polymeric multilayer and a simple method of making it where the multilayer includes a polymer layer with positive Δnth that can be disposed on polymeric substrate with negative Rth to form a multilayer having low Rth.